Malaria is the most significant vector-borne disease and affects mostly people living in the lesser-developed countries of tropical or sub-tropical regions. Changes in climate, rapid global transportation, immigration and invasion of exotic mosquito vectors bring a threat of introduction of the disease to developed nations. Sustainability of malaria control requires in part the discovery of therapeutic and prophylactic drugs, the development of vaccines, and control of vector mosquitoes. The long-term objective of this application is the production and utilization of strains of vector mosquitoes that are genetically refractory to the transmission of malaria parasites. These insects will be used to test the hypothesis that an increase in the frequency of a gene or allele that confers decreased vector competence to a population of mosquitoes will result in a reduction in the incidence and prevalence of malaria. We propose the development of strains of An. stephensi expressing specific effector molecules that interfere completely with the transmission of the important human malaria parasite, Plasmodium falciparum. To achieve this, the transgenic strains must express effector genes in specific mosquito tissues, the effector gene products must be present in quantities sufficient to disable all parasites, and the gene products must not impose too great a fitness burden. The SPECIFIC AIMS are to: 1) optimize expression of single-chain antibodies (scFv) that disable Plasmodium falciparum in the midgut and hemolymph of transgenic Anopheles stephensi;2) construct and test in parasite-challenge assays transgenic An. stephensi carrying single and multiple optimized scFvs for their ability to prevent parasites from infecting midguts and salivary glands;and 3) evaluate the fitness of strains of An. stephensi carrying one or more transgenes expressing antiparasite scFvs relative to control laboratory colonies using life-table parameters.